The plot for the movie is *not* based on the novel The Killer Elite (AKA Monkey in the Middle) by Robert Rostand (a nom de plume of script writer and author Robert Syd Hopkins), which was made into a Sam Peckinpah film starring James Caan. It is instead based on the controversial novel The Feather Men by Sir Ranulph Fiennes and is "based on a true story" (since Fiennes insists that it is true). Several elements from the book were altered to make the movie seem more believable to a movie-going audience.
Sir Ranulph Fiennes, the book's author, claims that a secret society called the "Feather Men", made up of retired and disabled SAS members, was operating in the shadows. They are called the "feather men" because their influence and intervention were subtle, like the touch of a feather. Their job was to protect SAS personnel and their families and avenge wrongs or harm done to them.
The hit squad in the novel was originally a communist Arab militant terror cell called 'the Clinic'. This was changed in the movie into a team of freelance assassins-for-hire controlled by an Angolan paymaster who runs a travel agency as a front.
The targets are three SAS troopers (one who is still serving and is a decorated war hero) who served in Oman in the 1970s. Their deaths must appear accidental to avoid reprisals. In the film, an added complication is that they must confess to being murderers before they are killed.
The patron in the novel was originally a rich Arab merchant from Dubai whose son died in combat while fighting in Oman. He is replaced in the movie with a disgraced Omani sheik dying of cancer who has to kill the men who killed his three eldest sons in order to restore his honor and return to his tribe.
The Battle of Mirbat, a siege in which eight SAS troopers with 100 assorted Firqat under training and 30 paramilitary askars (armed police) held off a force of 250 insurgents, is mentioned in passing in the film but never explained. One of the sheik's three sons were supposed to have been murdered there.
In the movie, the British Foreign Office is supposed to be in collusion with the sheik in order to guarantee oil leases on the sheik's land. They even force the "feather men" to back off with threats of imprisonment. However, the wealthy sheik is in exile and his son, a westernized playboy, shows no interest in returning to his homeland to claim his title. Therefore even if the sheik's plot is successful the British government will not have gained any leverage with the actual landholders, negating the premise. The book has no such subplot, as the "feather men" are seen as all-powerful.
Sir Ranulph Fiennes claims that the "feather men" saved his life from an assassination attempt by The Clinic. In the book 'The Clinic' tried to ambush him at his farmhouse in Exmoor, but the "feather men" ran them off. In the movie, he is a minor character who only survives because the assassin feels regret and only maims him.
Fiennes alleges in his novel that Major Mike Kealy, who died in 1979 while on a training exercise in the Brecon Beacons in Wales, was one of the "victims". He was supposedly drugged and then overcome and poisoned with insulin to induce diabetic shock. Why the assassins would need to track him down and poison him after drugging him is not explained. In actuality Kealy died of hypothermia during a freak sudden snowstorm because he was not wearing proper cold weather gear, refused to carry any before setting out, and discarded warm clothing he was given. If he had been dosed with insulin, he would have died of shock much earlier.(On a side note Kealy was an experienced SAS soldier and would have known better, this is what creates the doubt. No soldier, even a raw recruit, discards clothing when it is offered to him. In addition it is an offence on selection punishable by RTU "Return To Unit" to not have the full list of items in their packs)